A lens with an axial gradient index profile can cancel the spherical aberrations due to the refraction by the spherical lens surface. The conventional way of achieving this is to have a (essentially) linear dependence of the index in either the upper or the lower spherical cap. This is termed a "single-axial lens".
Such single-axial lenses are used in a variety of applications, for example, in binoculars. Such a configuration permits a reduction in the number of lens elements, and hence the weight of the binoculars.
Such lenses are made by a variety of what might be termed as "micro" processes. Such micro processes include diffusion into a lens blank of a refractive index-altering element of differing atomic number by immersion in a molten salt bath of silver chloride or by coating the lens blank with a thin layer that has a different index of refraction and then heating. Other techniques include implantation of ions into the surface of the lens blank and chemical vapor deposition of a species onto the surface thereof.
All such micro processes suffer from the fact that a substantial period of time is required to produce a desired gradient in the index of refraction and that in any event, such processing results in a gradient near the surface; gradients through relatively thick lens blanks (thickness greater than about fifteen millimeters) are not achievable in reasonable periods of time for commercial applications. Further, the maximum practically achievable index change is on the order of 0.05 for these micro lenses. This limits the use of these gradient index of refraction (GRIN) lenses. The present invention may be used to fabricate short focal length lenses, which cannot be fabricated by the above processes. These can be readily realized by the process of the invention, which can produce a wide range of desired changes in index of refraction.
Recently, so-called "macro" processes have been developed. By macro process is meant the use of bulk glass processing techniques, which can result in considerably thicker glass blanks having a gradient in index of refraction through the entire lens blank. Examples of such processing includes fusing layers of frits or plates together (each layer having a slightly different index of refraction). Typically, the top and bottom members are each of separate composition, with the intermediate members mixtures of the two compositions to give a desired gradient profile.
The macro processes yield lens blanks which have greater thicknesses than obtained by micro processes; dimensions of nearly 50 mm with a gradient along the entire thickness axis are easily fabricated. Further, differences in the index of refraction from one side to the other of 0.1 to 0.25 are routinely achievable, with differences approaching 0.5 attainable.
The macro processes permit fabrication of lens designs not heretofore available to the lens designer using the micro processes. For example, it is desired to correct not only spherical aberrations, but also chromatic aberrations. Further, it is desired to reduce the effects of temperature on the optical properties. The subject matter of the present application is directed to a single lens that has greatly reduced spherical and/or chromatic aberrations as well as greatly reduced dependence of the optical properties on temperature.
In Applied Optics, by Leo Levi, Vol. I, John Wiley & Sons, New York, (1968), pp. 490-493 is a description of cemented lens designs in which two (or three) different types of homogeneous glass are ground into the proper shape and then cemented together Of particular interest is the statement with regard to "one-radius doublet" that "[a] careful choice of glasses makes it possible to correct both chromatic and spherical aberrations, despite this severe restriction".
However, such a cemented lens is not structurally strong, nor is there a smooth variation in the index of refraction. This means that there is reflection within the lens, due to the discontinuous index change at the interface.
A lens design of the above type has been discussed by A. C. S. van Heel, "One Radius Doublets" in Optica Acta, Vol. 2, pp. 29-35 (1955). The discussion by van Heel points out the advantages of a single lens element that corrects both spherical and chromatic aberrations, but also makes note of the restricted possibilities available to the optical designer using the cemented process.
Accordingly, there remains a need for a monolithic (i.e., unitary) lens permitting correction of at least one of spherical and chromatic aberrations and thermal effects, particularly at short focal lengths.